1. Field of the Invention
The present invention relates generally to a method and system for deriving a vehicle speed representing value which can be used in wheel slip control, such as an anti-skid control, a traction control and so forth. More specifically, the invention relates to a derivation of vehicle speed representing value which precisely corresponds to an actual vehicle speed for accuracy in wheel slip control. Further specifically, the invention relates to a derivation of a vehicle speed representing value based on a rotation speed of a vehicular wheel.
2. Description of the Prior Art
As is well known, wheel slip control, such as anti-skid brake control and traction control, which controls driving torque on vehicular wheels for eliminating wheel-spin, is performed generally based on a wheel slippage. The wheel slippage is generally derived on the basis of a difference between a wheel rotation speed and a vehicle speed. Namely, anti-skid brake control is carried out to maintain wheel slippage around 10 to 20% for optimizing vehicular braking characteristics. On the other hand, in the traction control a, wheel-spinning condition is detected by detecting wheel speed exceeding a vehicle speed representative value.
Therefore, in order to perform precise wheel slip control, it is essential to detect vehicle speed accurately. Substantially high accuracy of vehicle speed detection can be made by means of a known lader systems, such as a dopper lader system. However, such lader system are expensive facilitating for wheel slip control. Therefore, in practice, the vehicle speed representing value has to be derived based on other parameter which can be obtained an inexpensive sensor or sensors.
Conventionally, the vehicle speed representing values have been derived on the basis of rotation speeds of vehicular wheels. For example, the Japanese Patent Second (examined) Publication (Tokko) Showa 41-17082 discloses an anti-skid brake control system with derivation of the vehicle speed representing value. In the disclosed procedure, one of the wheels which is rotatating at the highest speed is selected to take the rotation speed thereof, thus deriving the vehicle speed representing value. However, during abrupt deceleration of the vehicle, even the wheel speed of the highest speed wheel will not represent the vehicle speed. Therefore, in the conventional system, the vehicle speed representing value may be arithmetically derived by latching a highest wheel speed upon initiation of one cycle of wheel slip control operation and decreasing a constant value at every given interval for deriving assumed vehicle speed representing values to be used during abrupt deceleration.
Difficulty in performing accurate or precise anti-skid brake control arises when one of the wheels effect wheel-spin to accidentally increase wheel speed beyond the value corresponding to the vehicle speed. In such a case, errorneous detection is made while no wheel is skidding to initial anti-skid control. This causes expansion of braking distance due to lesser braking pressure to be exerted on the wheel.